Proton radiotherapy employs the depth dose characteristics and finite range of heavy charged particles to achieve conformal dose profiles that spare critical structures. These inherent advantages are not, however, fully utilized as there are presently no methods to monitor beam penetration during therapy and consequently margins are added in treatment plans that lead to inferior treatment outcomes. The proposed research aims to solve this problem by demonstrating the feasibility of measuring gamma-rays emitted during proton inelastic scattering as an in-situ proton range monitor. Preliminary results confirm previous reports and successfully correlate the measured signals with the proton depth-dose pro file. Experiments in this project will measure the spatial resolution and sensitivity attained using collimated radiation detectors. Monte Carlo calculations using the GEANT platform will also be performed to help optimize the experimental setup, interpret measured results and develop a conceptual design. Strategies to improve sensitivity by reducing the radiation background and isolating the characteristic gamma rays will be investigated through bolh measurements and simulations. A phantom developed at MGH containing bone and lung will be used to measure these signals under conditions representative of therapy. Successful completion of this project will establish the first practical method for proton range verification.